Various moving-medium type memory devices that use micro needles to read and write data have been proposed as replacements for hard disk drives and semiconductor memory devices such as CMOS devices. See, for example, U.S. Pat. No. 5,216,631 of Sliwa, Jr. and U.S. Pat. No. 5,373,494 of Kawagishi et al.
In a typical moving-medium type memory device, a large number of probes, for example, 100,000 probes, are arranged in a 2-dimensional array, and scan the surface of a recording medium. Each probe is composed of a cantilever probe arm and a micro needle. The probe arm is normally fixed at one end. The micro needle is mounted at or near the other end of the probe arm, remote from the fixed end, and extends perpendicularly to the probe arm towards the surface of the recording medium. Usually, the probe arms are driven by an electrostatic force to change the separation between the tips of the micro needles and the surface of the recording medium in the process of reading and writing the data.
Published International Application no. WO95/12932 of Saito and the inventor, and U.S. Pat. No. 5,600,137 of the inventor, disclose a moving-medium type memory device in which the probe arms are formed in part of a three-layer silicon structure composed of a silicon substrate, a buried silicon oxide layer, and silicon surface layer. The probe arms are formed by micromachining operations performed on the silicon surface layer.
The moving-medium type memory devices just described suffer from a number of problems.
First, the memory device conventionally includes an electronic circuit associated with each probe. The electronic circuit may include a data read/write circuit and a probe driving circuit, for example. The electronic circuit typically takes the form of an integrated circuit formed in and on the substrate near the probe. Such integrated circuits use a metal lead pattern to interconnect the various semiconductor components. The temperatures used in the processing to form the probe arms in the silicon surface layer are higher than those that can be tolerated by the metal lead pattern. Accordingly, when the memory devices are fabricated, the processing to form the metal lead patterns must be performed after the processing to form the probe arms in the silicon surface layer. This processing order means that the probe arms cannot extend over their respective electronic circuits. The electronic circuits must instead be located alongside the probes. This increases the area of the substrate occupied by the probes and their associated electronic circuits.
An important performance goal in moving-medium type memory devices is to increase the amount of data that can be read or written in one scan by increasing the density of the probes. Increasing the density of the probes also enables a moving-medium memory device of a given storage capacity to be made smaller, with a consequent lower manufacturing cost. However, locating the electronic circuits alongside the probes on the semiconductor substrate surface limits the probe density that can be achieved.
Second, the electronic circuits project from the semiconductor substrate surface towards the surface of the recording medium. The electronic circuits project by a much greater distance than the maximum deflection distance of the moving end of the probe arm on which the micro needle is mounted. Consequently, when the electronic circuits are fabricated in the same substrate as the probe arms, the maximum deflection distance of the moving end of the probe arm must be increased, or the micro needle must be lengthened to accommodate the height of the electronic circuits above the substrate surface. To increase the maximum deflection distance of the moving end of the probe arm, the drive voltage for the probe arm must be increased. This increases the power consumption of the moving-medium type memory device. Alternatively, the micro needle can be lengthened to accommodate the height of the electronic circuit, but this cannot be done using currently-available process technologies. Even if the micro needle could be lengthened, a longer micro needle would be more susceptible to damage.
The problem just described can be partially solved by providing the probe arm with an auxiliary electrode. The auxiliary electrode has an electrode surface disposed parallel to the semiconductor substrate surface and located closer to the surface of the recording medium than the probe arm. The auxiliary electrode enables the probe arm to be driven by a lower voltage, and does not require significant lengthening of the micro needle.
However, even when the probes are fitted with an auxiliary electrode, the height to which the electronic circuits can project above the surface of the substrate is limited. The height limitation is such that the interconnections of the electronic circuits cannot be formed using multiple layers. It is desirable to be able to form the electrical interconnections using multiple layers, since this would enable the area of the substrate occupied by the electronic circuits to be reduced. This would result in a higher probe density and a smaller moving-medium type memory device.
Finally, forming the probes by micromachining techniques in the upper layer of a three-layer semiconductor structure can produce probes that are subject to warping. Warping occurs as a result of the high temperatures involved in the probe fabrication process leaving high residual stress in the material of the probe. The probe warping changes the gap between the surface of the recording medium and the tip of the micro needle mounted on the probe arm. This gap having an incorrect value causes data read/write errors. To prevent probe warping from causing such errors, the electronic circuit must include an additional circuit to control the gap or to control the contact pressure between the surface of the recording medium and the tip of the micro needle. This additional circuit makes it harder to increase the probe density of the moving-medium type memory device.
A probe apparatus is desired that achieves the above-mentioned goals of increasing the probe density and reducing size without increasing the maximum deflection of the moving end of the probe arm and without increasing the length of the micro needle, that allows multi-layer interconnections to be used in the electronic circuits, and that prevents read/write errors by using probes that do not warp.